CONCRETIONARY STRUCTURE. 



99 



107. The concretions in sandstones are usually spheres or sphe- 

 roidal, while those in argillaceous layers are flattened disks. 



Fig. 85. 



In fig. 86 the lower sandstone layer (1) has no concretions ; the other (3) con- 

 tains spherical concretions; in the upper layer (4), an argillaceous sandstone, the 

 concretions are somewhat flattened and coalescent ; in the shaly layer (2) they 

 are very much flattened, and in its lower part coa- 

 lescent. 



Concretions sometimes take fanciful or imitative 

 shapes; and every geologist has had petrified tur- 

 tles, human bones, skulls, and toads brought him, 

 which were only examples of the imitative freaks, 

 of the concretionary process. The turtles are usually 

 what are mentioned as septaria on page 95. Occa- 

 sionally concretions take long cylindrical forms, from 

 consolidation around a hole bored by a worm or mollusk, the hole giving pass- 

 age to the concreting ingredient ; or they derive their form from some rootlet 

 or stem of a plant, in which case they are often branched. 



A radiated arrangement is common when no distinct concretions are formed, 

 as with quartz crystals in irregular cavities. 

 Sometimes different points become centres of radia- 

 tion, producing a blending of distinct radiations, as 

 in fig. 87. 



Very many of the mineral species shoot into stellar 

 and globular radiated crystallizations. Others, like 

 pyrites, readily collect in balls or nodules around a 

 foreign body as a nucleus, or, if none is at hand, 

 around the first molecule of pyrites that commences 



the crystallization. This tendency in nature to concentric solidification is so 

 strong that no foreign nucleus is needed. The iron-ore of coal-regions is 

 mostly in concretions in certain layers of the Coal measures. The rounded 

 masses often lie imbedded in the clayey layer, or are so numerous as to coalesce 

 into a solid bed. 



108. b. The Jointed Structure— Joints in rocks are planes of fracture 

 or division cutting directly across the stratification and extending 



Fig. 87. 



